A conventional detection device for reducing of the air pressure of tires uses a principle that since the outer diameter of a tire (the dynamic load radius of a tire) is reduced more than that of a tire having a normal inner pressure when the pressure of a tire is reduced, a rotational angular velocity is increased as compared with other normal tires. For example, a method of detecting the lowering of an inner pressure from the relative difference of the rotational angular velocity of a tire uses as a judgment value;    DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%) (Refer to Japanese Unexamined Patent Publication No. 305011/1988).
Where F1 to F4 are the rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
When DEL value exceeds a preliminarily set alarm threshold by calculating the DEL value, further carrying out requisite corrections such as cornering correction, and deleting data unsuitable for judgment, it is judged that the reduced pressure of a tire is generated, and an alarm is informed to a driver. However, even if the reduced pressure is the same 30%, the size of the above-mentioned DEL value differs depending on the kind of a tire and the like. Accordingly, the reduced pressure sensitivity being the change rate of a dynamic load radius caused by reduced pressure which a respective tire has is respectively measured for preliminarily set tires (tires for summer and tires for winter) in order to determine the above-mentioned threshold. By setting the threshold at intermediary reduced pressure sensitivity, it is designed not to generate a great difference in detection sensitivity, even if the tires were also replaced. For example, when tires having 2-fold difference in the reduced pressure sensitivity are mounted on the same vehicle, an alarm threshold is set so that a high sensitive tire alarms at a reduced pressure of 20% and a low sensitive tire alarms at a reduced pressure of 40%.
Since a conventional device detects the reduced pressure of a tire utilizing so-called reduced pressure sensitivity that the dynamic load radius of a tire is lessened by reduced pressure, the judgment of the reduced pressure is carried out by difference between one of the sum of rotational angular velocities and the other of the sum of the rotational angular velocities which are situated at the diagonal of four wheels. Therefore, the simultaneous reduced pressures of two front wheels, the simultaneous reduced pressures of two rear wheels, and the simultaneous reduced pressures of whole wheels cannot be detected. Accordingly, there are problems that driving a car without knowing the reduced pressures causes the deterioration of fuel cost caused by the increase of rolling resistance of tires, and further causes burst.
Further, since alarm at a reduced pressure of 30% is an essential requisite by Law Regulation in the North America, there is a problem that the approved range of the reduced pressure level for alarming must be lessened so as to alarm between a reduced pressure of 15% and a reduced pressure of 30%. Further, there is a problem that a case unable to be corresponded with a lesser approved range is generated when set tires for one grade of a car have a wide range. The information of the reduced pressure sensitivities of tires mounted is required for solving these problems, but it is ambiguous in reality what type of tires a user mounts, therefore the reduced pressure sensitivities of tires currently mounted cannot be known.